Cardiac perfusion imaging with SPECT is one of the most widely used diagnostic tests in the assessment of coronary artery disease. Typically the images are quantified and compared to a data base of normal values. However, attenuation of the relatively low energy photons emitted by Tl- 201 or Tc-99m (the two principle agents used) distorts the images obtained. This distortion may increase the standard deviation of the normal data base, thereby decreasing the sensitivity and specificity of the test. This study proposes to investigate the degree to which attenuation correction might decrease the normal database variability, thereby increasing sensitivity and specificity of the test. To accomplish this, simulations were performed, based on PET images. In these simulations, actual cardiac PET images were forced to have "normal" uptake of tracer, by setting all pixels within the myocardium to a constant value. These images were then projected through the measured, true, attenuation map, and reconstructed using the standard SPECT filtered back projection methods. The SPECT reconstructed images could then be compared with the known uniform background. Preliminary data were presented orally in June 1995. It is hoped that data can be obtained to examine this effect separately for men and for women (who are purported to have significantly different attenuation patterns due to breast tissue).